ARENICOLA, 189 



series enable the animal to lengthen or shorten its body, like a concer- 

 tina. But this by itself would not effect movement of the whole body. 

 Farther provision is usually found in hooks, suckers, or setce, which 

 catch the ground or surrounding medium and prevent movement in one 

 direction, usually backwards. By this means all the expansion or 

 lengthening and shortening or contraction must take place forwards and 

 progress is therefore rapid. We may emphasise in this arrangement the 

 motor function performed by the coelom and its fluid, which may be 

 compared with the more specialised condition in the proboscis of 

 BalanoglossHS and the ambulacral system of Asterias. 



The excretory organs consist of six pairs of nephridia (in 



segments 4 to 9 inclusive). These are wide tubes which 



open into the coelom by large funnels or 



nephrostomes and to the exterior by small ne- 



phridiopores. A patch of coelomic epithelium covering each 



Fig. 115. — A Nephridium of Arenicola. 



(After AsHWORTH and Gamble.) 



J'"""^!- \^ __Blood-Ple> 



Gonadiai 

 Filaments. 





Vesicle or Bladder. 



of the nephridia, except the first, gives rise to the sexual 

 R d ti elements, which lie free in the nutritive 

 ' coelomic fluid and pass to the exterior by the 

 nephridia. Arenicola is dioecious and breeds in spring 

 and summer. It develops by a free larval form allied to 

 the trochophore. 



[I ABLE. 



